A. Field of the Invention
The present invention relates generally to encryption systems. More particularly, the present invention relates to the use of encryption devices in mobile communications.
B. Description of the Related Art
The use of encryption algorithms in cellular and personal communications have been constrained for various reasons. First, the lack of computing power in mobile stations, such as a mobile telephone, may limit the use of computationally intensive encryption algorithms, such as public key algorithms. Second, due to the high bit error rate of wireless channels, encryption algorithms may propagate errors that further deteriorate the quality of data transmission. Third, the shortage of bandwidth at uplink channels (e.g., from a mobile station to a base station) may make ciphers at low encryption (or decryption) rates unacceptable. Moreover, random delays in encryption or decryption may be present, which further reduces the desirability of encryption algorithms in cellular and personal communications.
To handle these issues, the European Group Special Mobile (GSM) adopted a hardware implemented stream cipher known as A5. The A5 stream cipher consists of three linear feedback shift registers of total length 64 bit that are mutually clocked in a stop-and-go manner.
Unlike a block cipher that accumulates data into a block before ciphering, a stream cipher may directly process messages of arbitrary size by ciphering individual elements, such as bits or bytes. Stream ciphers may be used in secure communications and commonly include a keystream generator whose output sequence is added modulo 2 to the plaintext bits producing a random-looking sequence, which is also referred to as a pseudo-random sequence.
Many kinds of keystream generators have been proposed including combination generators, filter generators, and clock-controlled shift registers. These keystream generators include one or more linear feedback shift registers. A combination generator may consist of several linear feedback shift registers whose outputs are combined by a nonlinear Boolean function. A filter generator may consist of a single linear feedback shift register and use a nonlinear Boolean function to filter the contents of the shift register. Clock-controlled shift registers may irregularly clock certain parts of the keystream generator in order to achieve nonlinear effects.
To solve the synchronization problems inherent in stream ciphers, the A5 stream cipher combines a 64-bit secret session key with a 22-bit frame number to generate a 64-bit subkey for each frame. However, one disadvantage of A5 stream cipher is that its hardware implementation may add additional cost to the fabrication of cellular and personal communications units. Another disadvantage of A5 stream cipher is that the cost to readily modify the A5 stream cipher may be high in the event that a modification becomes necessary to overcome a known vulnerability of the A5 cipher.